1. Field of the Invention
The subject invention is directed generally to methods and materials for treating conditions and disorders associated with phenylalanine metabolism and, more particularly, to methods and materials for treating phenylketonuria.
2. Description of the Related Art
Phenylketonuria (PKU) is hyperaminoacidemia of phenylalanine (Phe) associated with an inborn error of phenylalanine metabolism, mutation of the gene encoding phenylalanine 4-hydroxylase (PAH), which converts phenylalanine to tyrosine. In some cases, an additional metabolic defect occurs in the synthetic pathway of either dihydropteridine or tetrahydrobiopterin (BH4), phenylalanine 4-hydroxylase co-factors, contributing further to the hyperphenylalaninemia (HPA). Whereas a normal plasma phenylalanine level is approximately 0.05 mM (1), untreated “classic” PKU patients have plasma phenylalanine levels above 1 mM, e.g., plasma phenylalanine levels of from about 1 mM to about 2.5 mM or more, and, although treatment with a low-phenylalanine diet has a goal of reducing plasma phenylalanine to below 0.3 mM, this is difficult to attain due to dietary compliance problems. In the United States, about 1 in 10,000 babies are born with PKU.
The excessive levels of plasma phenylalanine observed in PKU combined with the relatively high affinity of phenylalanine for binding sites on carrier protein of the neutral amino acid transport system in the blood-brain barrier (BBB) leads to (i) accumulation phenylalanine Phe and its neurotoxic metabolites, e.g., phenylpyruvate, phenylacetate, phenyllactate, in the brain and (ii) depressed levels of non-phenylalanine neutral amino acids entering the brain, resulting in disturbed brain development and function, since key cerebral pathways of metabolism, e.g., synthesis of neurotransmitters, require precursor amino acids, such as tyrosine. This depression is pronounced for tyrosine, which is low in the plasma supply due to the PKU metabolic error in the enzyme responsible for converting phenylalanine to tyrosine. Current thought is that the neurological deficits of PKU are due predominantly to the depression of levels of non-Phe neutral amino acids entering the brain (2).
Although a diet low in phenylalanine can reduce plasma phenylalanine levels in “classic” PKU below 0.3 mM and ameliorate the mental retardation associated with untreated PKU, dietary compliance can be problematic and can become particularly problematic as PKU patients reach adolescence, leading to a rise in plasma phenylalanine levels and to both loss in intelligence and white matter changes in the brain. In addition to requiring patient compliance, therapies based on dietary restriction also requires that the patient know the level of phenylalanine present in a particular food. Moreover, nutritional deficiencies can also result from phenylalanine-restricted diets.
Alternative treatments have thus been developed. For example, to overcome suspected depletion of the neurotransmitters dopamine and serotonin, PKU patients have been treated with the neurotransmitter precursors tyrosine and tryptophan (3). To reduce influx of phenylalanine into the brain, a supplement of branched chain neutral amino acids containing valine, isoleucine, and leucine, was administered to older PKU patients (4), who reported significant improvement in behavioral deficits. It was proposed that the addition of the neurotransmitter precursors, tyrosine and tryptophan (2) to Berry's supplement (4), should lead to further improvement. However, efficacy of these dietary amino acid supplement treatments has been controversial.
In view of the above, there is a recognized need for methods and compositions for treating phenylketonuria and other conditions and disorders associated with phenylalanine metabolism. Specifically, the prior art is deficient in methods of treating phenylketonuria by administering phenylalanine 4-hydroxylase solely or in combination with one or more other enzymes that improve its activity in metabolizing phenylalanine. The present invention fulfills this long-standing need and desire in the art.